The present invention is related to co-owned U.S. Pat. No. 4,994,671 to Safinya et al., No. 5,167,149 to Mullins et al., 5,201,220 to Mullins et al., No. 5,266,800 to Mullins et al., and No. 5,331,156 to Hines et al., all of which are hereby incorporated by reference herein in their entireties.
1. Field of the Invention
The present invention relates to the analysis of downhole borehole fluids. More particularly, the present invention relates to apparatus and methods for the downhole compositional analysis of gas in a geological formation through the use of spectroscopy.
2. State of the Art
Techniques for the qualitative and quantitative analysis of gas, liquid, and solid samples are well known. For example, as disclosed in U.S. Pat. No. 4,620,284 to R. P. Schnell, a helium-neon laser is used to provide photons of a 0.633 micron wave length which are directed at a sample flowing through a pipeline in an oil refinery. The resulting Raman scattering (scattering of light by molecular excitation) which comprises scattered light at different wavelengths than the incident light is then measured, and the measured spectrum is compared with previously obtained reference spectra of a plurality of substances.
In U.S. Pat. No. 4,609,821 to C. F. Summers, especially prepared rock cutting containing at least oil from an oil-based mud are excited with UV radiation with a 0.26 micron wave length. Instead of measuring the Raman spectrum as is done in the aforementioned Schnell patent, in accord with the Summers disclosure, the frequency and intensity of the resulting excited waves (fluorescence) which are at a longer wavelength than the incident radiation are detected and measured. By comparing the fluorescent spectral profile of the detected waves with similar profiles of the oil used in the oil-based mud, a determination is made as to whether formation oil is also found in the rock cuttings.
While the Summers and Schnell disclosures may be useful in certain limited areas, it will be appreciated that they suffer from various drawbacks. For example, the use of laser equipment in Schnell severely restricts the environment in which the apparatus may be used, as lasers are not typically suited to harsh temperature and/or pressure situations (e.g., a borehole environment). Also, the use of the Raman spectrum in Schnell imposes the requirement of equipment which can detect with very high resolution the low intensity scattered signals. The use by Summers of light having a 0.26 micron wavelength severely limits the investigation of the sample to a sample of nominal thickness. In fact, the Summers patent, while enabling a determination of whether the mud contains formation oil, does not permit an analysis of formation fluids in situ, and has no sensitivity to water.
Those skilled in the art will appreciate that the ability to conduct an analysis of formation fluids downhole is extremely desirable. With that in mind, the assignee of this application has provided a commercially successful borehole tool, the MDT (a trademark of Schlumberger) which extracts and analyzes a flow stream of fluid from a formation in a manner substantially as set forth in co-owned U.S. Pat. No. 3,859,851 to Urbanosky U.S. Pat. No. 3,780,575 to Urbanosky which is hereby incorporated by reference herein in its entirety. The OFA (a trademark of Schlumberger), which is a module of the MDT, determines the identity of the fluids in the MDT flow stream and quantifies the oil and water content based on the previously incorporated related patents. In particular, previously incorporated U.S. Pat. No. 4,994,671 to Safinya et al. provides a borehole apparatus which includes a testing chamber, means for directing a sample of fluid into the chamber, a light source preferably emitting near infrared rays and visible light, a spectral detector, a data base means, and a processing means. Fluids drawn from the formation into the testing chamber are analyzed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information accordingly (and preferably based on the information in the data base relating to different spectra), in order to quantify the amount of water, gas, and oil in the fluid. As set forth in previously incorporated U.S. Pat. No. 5,266,800 to Mullins, by monitoring optical absorption spectrum of the fluid samples obtained over time, a determination can be made as to when a formation oil is being obtained as opposed to a mud filtrate. Thus, the formation oil can be properly analyzed and quantified by type. Further, as set forth in the previously incorporated U.S. Pat. No. 5,331,156 to Hines et al., by making optical density measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified.
While the Safinya et al., Mullins, and Hines et al. patents represent great advances in downhole fluid analysis, and are particularly useful in the analysis of oils and water present in the formation, they do not address in detail the gases which may be plentiful in the formation. The issue of in situ gas quantification is addressed in the previously incorporated U.S. Pat. Nos. 5,167,149 to Mullins et al., and 5,201,220 to Mullins et al., and in O. C. Mullins et al., "Effects of high pressure on the optical detection of gas by index-of-refraction methods", Applied Optics, Vol. 33, No. 34, pp. 7963-7970 (Dec. 1, 1994) which is also incorporated by reference herein in its entirety, where a rough estimate of the quantity of gas present in the flow stream can be obtained by providing a gas detection module having a detector array which detects light rays having certain angles of incidence. While rough estimates of gas quantities are helpful, it will be appreciated that compositional analysis of the gas would be more useful. In particular, gas analysis can be useful in determining which zones of a formation to produce, as gas zones with higher hydrocarbon content and with higher BTU content are more valuable than gas zones with lesser hydrocarbon content. In addition, it would be advantageous to be able to control the BTU content of the gas being produced without undergoing gas separation and recombination uphole, but rather by controlling quantities being produced from different locations in the borehole with advance knowledge of their respective BTU contents. Furthermore, downhole gas analysis could reveal the presence of noxious gases such as H.sub.2 S. Since H.sub.2 S is reactive with logging tool metals and is also reactive with basic materials contained in water based mud filtrate, analysis of samples carried to the surface often underestimate the noxious gas content of the samples.
While techniques such as chromatography are routinely used for the analysis of gas in laboratories, the use of gas chromatography downhole is impractical due to several reasons. First, gas chromatography requires the use of a carrier gas flow stream whose volume far exceeds the sample gas volume. The handling of carrier gas within closed volumes of wireline tools represents a major problem especially considering that the formation gas pressures far exceed typical operating pressures of gas chromatography equipment. Second, handling of sample gas in gas chromatography equipment is difficult as very small volumes are used, and it is difficult to collect and transfer such small volumes from a large flow stream and guarantee representative samples. Third, the standard detectors for hydrocarbons in gas chromatography are a flame ionization detector and a thermal detector, both of which are impractical downhole because of the requirement of the maintenance of a stable flame downhole, and the wide variation in temperatures downhole. Finally, gas chromatography requires discrete measurements lasting several minutes which is undesirable in wireline applications. On the other hand, while spectroscopy has been used downhole for distinguishing between oil and water (in the near infrared spectrum), and for distinguishing among oils (in the visible spectrum), downhole spectroscopy has not been suggested for distinguishing between different hydrocarbon gases such as methane (CH.sub.4), ethane (having methyl components (CH.sub.3)), and higher hydrocarbons which contain methylene (CH.sub.2) for several reasons. First, because the density of a gas is a function of pressure, and because downhole pressures can vary by a factor of thirty or more, the dynamic range of the gas densities likely to be encountered downhole is extremely large. As a result, the dynamic range of the spectral absorption at frequencies of interest is also extremely large such as to make a measurement unfeasible; i.e., the sensitivity of the downhole spectroscopy equipment is typically incapable of handling the large dynamic ranges that are encountered. Second, due to fact that the condensed phase of hydrocarbon (oil) has a much higher density at downhole pressures than the gas phase, a thin film of liquid on the OFA window can yield significant absorption. Thus, interpretation of the results would yield a determination of a rich gas mixture, where no or little amounts of hydrocarbon gas was actually present. Third, the type of spectral analysis typically done uphole to distinguish among hydrocarbon gases cannot be done downhole. In particular, in uphole applications, individual gas constituents are detected by modulating a narrow band source on and off of mid-infrared absorption lines of the gas, where a resulting oscillation in absorption at each modulation frequency would indicate a positive detection of a particular gas. However, at the high pressures encountered downhole, not only are the narrow gas absorption spectral lines merged, but mid-infrared spectroscopy is hindered by the extreme magnitude of the absorption features. Fourth, spectrometers are typically sensitive to changes in temperature, and elevated temperatures encountered downhole can induce spectral changes of the gas sample, thereby complicating any data base utilized.